The industry is starting to recognize this as a systematic problem. For reasons currently unknown, BGAs assembled to ENIG-plated boards that are exposed to a second reflow operation periodically experience a severe weakness in the solder joint or, in more extreme cases, dewetting. If you could send me some images of this defect, I can give you some feedback if this is indeed your affliction.
Dr. Craig D. Hillman
CEO & Managing Partner, DfR Solutions
Dr. Hillman's specialties include best practices in Design for Reliability, strategies for transitioning to Pb-free, supplier qualification, passive component technology and printed board failure mechanisms.
I have seen this type of problem before. Although you may or may not be mechanically stressing it, it is likely it is being stressed during handling, packaging, shipping or when your customer is handling it. Unless it is a backplane and extremely rigid this would likely be the culprit. The solution is handling and storing the boards on a flat surface. ESD safe plastic trays are the best way, as the board is fully supported on a flat surface. The operators are handling the tray, not the board, the board will never see stress. You and your customer need to use the same method for handling. The prior industry method before trays and tray carts was carts or racks with slotted side panels and the boards would hang from two edges and sag in the middle and frequently fracture solder joints on BGA's and or circuits. These older rack methods are still around, but the industry is changing. You can get trays and tray carts from a variety of companies, including mine. I hope this solves the problem for you.
Ken Bliss
President & CEO - Retired, Bliss Industries, Inc.
Retired - Mr. Bliss has 20+ years experience creating process methods that improve profitability by maximizing hidden unused capacity and throughput. Ken has expertise in all areas of manufacturing specializing in electronics assembly.
You may want to try a step up stencil in the BGA area; step up to 8 mil thick. This may help with large BGA's
Bill Coleman
Vice President Technology, Photo Stencil
For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil, working closely with customers to understand their printing requirements. His efforts have resulted in several new stencil products.
In my opinion....40% of the task is materials/solderability....20% is mechanics (alignment/placement/pcb+component handling)....40% is thermal profile. I would look at making sure your chemistry is correct....and also your thermal profiling. Are you warping the pcbs during reflow? If so, a lot of stress will be induced in your assembly...which will lead to subsequent failures. You may need to slow the preheating rate down, allowing heat to evenly perculate through the whole structure, to reduce the warping. Also the reflow temperature + soak times need to be sufficient to allow the bonding to occur...your solder manufacturer will have guidelines to follow. Only my thoughts...
Roger Gibbs
Managing Director, PDR
Twenty years ago Mr. Gibbs, Managing Director of PDR, produced the first rework system based on Focused Infra-Red light energy. PDR now has a 3,500-strong global installation list with top OEMs and EMS providers.
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